Japanese Patent Unexamined Publication No. 59-20825 discloses a prior art auto-tensioner (shown in FIG. 12 generally at A) which has an idler pulley 4 and is adapted to adjust the tension of a toothed timing belt 3 put around a pulley 2 of a crank shaft 1 mounted on an automobile engine E and a cam shaft pulley (not shown). This autotensioner A is of a hydraulic type and comprises a housing 5, a cylinder 6 mounted in the housing and a plunger 7 slidably mounted in the cylinder. The cylinder 6 and the plunger 7 define a hydraulic chamber 8 in which a check valve 9 and a spring 10 are mounted. A bellows 11 seals oil in the chamber 8. The timing belt 3 is automatically tension-controlled by the auto-tensioner. This prior art auto-tensioner is hydraulically operated and requires many parts and has to be accurately assembled. It is also expensive and troublesome to assemble.
An object of the present invention is to provide an auto-tensioner for belt which obviates the abovesaid shortcomings.